1. Field of the Invention
The present invention relates to recombinant nucleic acid molecules and recombinant fusion proteins, and more particularly to Shiga-like toxin-vascular endothelial growth factor fusion proteins and recombinant DNA molecules coding for such fusion proteins. The present invention also relates to bacterial vectors containing the above recombinant nucleic acid molecules, methods of producing the above fusion proteins, and their use in therapeutic treatments.
2. Description of the Related Art
Angiogenesis is a tightly controlled process of growing new blood vessels (see, Folkman & Shing, 1992; Hanahan, 1997, for reviews). Under normal circumstances angiogenesis occurs only during embryonic development, wound healing and development of the corpus luteum. However, angiogenesis occurs in a large number of pathologies, such as solid tumor and metastasis growth, various eye diseases, chronic inflammatory states, and ischemic injuries (see, Folkman, 1995, for review). Thus, growing endothelial cells present unique targets for treatment of several major pathologies.
The crucial positive regulator of angiogenesis is vascular endothelial growth factor (VEGF) also known as vascular permeability factor (see, Neufeld, et al, 1999 for reviews). VEGF is a secreted dimeric glycoprotein that, as a result of alternative splicing, may consist of polypeptides with 121, 145, 165, 189 and 206 amino acid residues. VEGF is expressed by normal and tumor cells and the control of VEGF expression appears to be regulated on several levels (see, Claffey & Robinson, 1996, Veikkola & Alitalo, 2000, for reviews). Expression of VEGF is upregulated in response to hypoxia and nutritional deprivation suggesting a feedback loop between tumor and metastasis growth and the ability of tumor cells to induce host angiogenic responses.
The action of VEGF on endothelial cells is mediated by tyrosine kinase flt-1 and KDR/flk-1 receptors, also known as VEGFR-1 and VEGFR-2 (see, Terman, & Dougher-Vermazen, 1996; Veikkola, et al, 2000, for review). These receptors are preferentially expressed on endothelial cells. There are reports that endothelial cells at the sites of angiogenesis express significantly higher numbers of KDR/Flk1 receptors than quiescent endothelial cells (Brown, et al., 1993, 1995; Plate, et al., 1993; Detmar, et al., 1994; Couffinhal, et al., 1997). The receptors are single span transmembrane protein tyrosine kinase that belong to the immunoglobulin superfamily and contains seven Ig-like loops in the extracellular domain and shares homology with the receptor for platelet-derived growth factor. VEGF binding to these receptors induces receptor dimerization followed by tyrosine phosphorylation of the SH2 and SH3 domains in the dimer (see, Neufeld, et al., 1994 for review). KDR/Flk1-VEGF complex is internalized via receptor-mediated endocytosis (Bikfalvi, et al., 1991).
Several groups reported that targeting of either VEGF or KDR/flk-1 inhibits angiogenesis and angiogenesis-dependent processes (Kim, et al., 1993; Millauer, et al., 1994; Saleh, et al., 1996; Aiello, et al., 1995). On the other hand, direct injection of VEGF or a plasmid encoding VEGF into ischemic tissues in a model system promoted development of microvasculature and improved recovery after ischemic injury or balloon angioplasty (Asahara, et al., 1996). Taken together, these results leave little doubt that VEGF and KDR/Flt1 play crucial roles in angiogenesis. Although these experiments provided a “proof-of-principle” that VEGF-toxin conjugates or fusion proteins may work in vivo, further development of DT-VEGF constructs is doubtful, because of the renal and liver toxicity of DT-containing fusion proteins (see, for example, Vallera et al., 1997).
Since VEGF binds specifically to endothelial cells, this growth factor provides a unique opportunity for targeted drug delivery to the sites of angiogenesis. It was demonstrated that catalytically active forms of diptheria toxin covalently linked or fused via recombinant DNA technology to recombinant VEGF165 and/or VEGF121 are selectively toxic against cells expressing KDR/flk-1 receptors and also suppressed angiogenesis in vivo (Ramakrishnan, et al., 1996; Olson et al., 1997; Arora, et al., 1999).
It is advantageous to use VEGF for targeting toxins that are “natural killers” of endothelial cells. Shiga-like toxin 1 produced by E. coli O157:H7 is such a “natural killer” for endothelial cells. Damage to endothelial cells caused by Shiga-like toxins 1 plays a causative role in the pathogenesis of hemorrhagic colitis (HC) and hemolytic uremic syndrome (HUS) induced by E. coli O157:H7 (Obrig, et al., 1987, 1993; Richardson, et al., 1988; Kaplan, et al., 1990).
Shiga-like toxin 1 (SLT-1) is composed of a single copy of a 32 kDa A-subunit associated with a ring shaped pentamer of receptor-binding 7 kDa B-subunits. B-subunits bind SLTs to the cellular receptor globotrioaosylceramide known as Gb3 (Obrig et al., 1993). This receptor is found on many cell types including endothelial cells (Obrig et al., 1993). After binding to the cell surface receptor, SLT is endocytosed and A-subunit is cleaved into A1 (27.5 kDa) and A2 (4.5 kDa) forms that are linked by disulphide bond (Olsnes et al., 1981). Processed A subunit is a specific N-glycosidase that inactivates ribosomes by cleaving off a single adenine residue in the position 4324 from 5′ terminus of 28S rRNA of 60S ribosome subunit (Saxena et al., 1989). The cleavage of A4324 from 28S rRNA inactivates ribosomes by inhibiting binding of the elongation factor (EF-1)/aminocyl-tRNA complex to ribosomes, resulting in the inhibition of the protein synthesis. As with other ribosome-inactivating agents, the subsequent cytostatic and cytotoxic effects might arise as a cellular response to inactivation of a relatively small proportion of ribosomes through ribotoxic stress response (Iordanov et al., 1997). Alternatively, cytostatic and cytotoxic effects might arise as a cellular response to a massive collapse of protein synthesis due to inactivation of a large number of ribosomes. It is important that the unprocessed, full length A subunit as well as various truncated A subunits retain significant N-glycosidase activity (Haddad, et al., 1993; Al-Jaufy, et al., 1994, 1995). Furthermore, fusion proteins containing unprocessed, full length A subunit as well as various truncated A subunits fused to N-terminus of CD4 retain N-glycosidase activity and are cytotoxic for cells expressing HIV-1 gp120-gp41 complex (Al-Jaufy, et al., 1994, 1995).
Since Shiga-like toxin is a “natural” killer of endothelial cells it is advantageous to deliver enzymatically active full-length, truncated or mutated A subunit into endothelial cells in order to inhibit their growth and/or kill them. To avoid damage to other cell type the enzymatically active full-length, truncated or mutated A subunit should be delivered into target cells by endothelial cell specific growth factor such as VEGF. Therefore, it is an object herein to provide effective recombinant DNA methods for the production of fusion proteins containing enzymatically active full-length, truncated or mutated A subunit fused to full-length, truncated or mutated VEGF that retain ability to bind to VEGF receptors.